Three areas of research on murine leukemia viruses (retroviruses) involving their 1) processing of viral RNAs, 2) regulation of provirus expression, and 3) oncogenicity are included here. Analysis of 38S viral RNAs by the S1 heteroduplex mapping technique will be carried out to determine whether different 38S RNAs exist which differ in their splice patterns. Further studies of structural and functional differences between 38S genomic and messenger RNAs will involve the study of a unique replication-defective Moloney murine leukemia virus (M-MLV) cell clone, NX-3. Studies on the transcriptional regulation of M-MLV proviruses in NRK-4 cells will include the analysis of viral DNA and RNA in cells transfected with DNA from molecular clones of these proviruses. These studies will also examine how the chromosomal conformation of the cellular DNA sites of integration is related to the DNAse I sensitivity of the provirus after integration. Finally, the study of leukemogenesis in the AKR mouse will include the analysis of MLV provirus integration sites and mRNA expression in thymic lymphomas by agarose gel electrophoresis, transfer to nitrocellulose filters and hybridization to viral specific probes. These studies will test different hypotheses involving the role of MLVs in the development of murine lymphocytic leukemia.